In-line adsorber system

ABSTRACT

A modified in-line adsorber system capable of meeting the California ultra-low emission vehicle (ULEV) standard using a combination of burn-off catalyst and a modified adsorber constructed with an open flow region of substantially unobstructed flow having a hole or a region of larger cell openings to increase the amount and rate of contact between the exhaust gas and the burn-off catalyst, and thereby reduce the light-off time of the burn-off catalyst. For best effect, the open flow region of the adsorber is positioned along the exhaust stream between the engine and the burn-off catalyst as defined by the exhaust flow path of least resistance.

This is a division of application Ser. No. 08/484,617, filed Jun. 8,1995now U.S. Pat. No. 5,787,707.

BACKGROUND OF THE INVENTION

The invention relates to an in-line adsorber system for an internalcombustion engine, capable of meeting the California ultra-low emissionvehicle (ULEV) standard, using a modified adsorber construction.

Internal combustion engines emit large amounts of unburned hydrocarbonsduring the cold start of an engine due to the rich fuel mixture used insuch engines, and the necessarily incomplete combustion at start-up.This emission of unburned hydrocarbons continues until the main catalystreaches its "light-off" temperature, at which point the catalyst beginsto convert the hydrocarbons to harmless gases. The typical catalyticlight-off time for most internal combustion engine systems is around 50to 120 seconds, during which time significant amounts of hydrocarbonsare emitted into the atmosphere. The actual catalytic light-off time forany system will depend on the position of the catalyst relative to theengine, as well as the noble metal loading. The temperature of thecatalyst is elevated by contacting it with the high temperature exhaustgases from the engine, and continuous contact with those gases alongwith the exothermic nature of the oxidation reactions occurring at thecatalyst combine to maintain the catalyst at an elevated temperature.

While catalytic converters are well known for reducing oxides ofnitrogen (NOx), and oxidizing hydrocarbons and carbon monoxide fromautomobile exhaust, these reactions typically take place after thecatalyst has attained its light-off temperature. That is, attemperatures generally in the range of 200-350° C. depending on suchfactors as the noble metal loading and aging of the catalyst. Typicallyhowever, seventy to eighty percent of hydrocarbon emissions fromautomotive vehicles are emitted during about the first minute of engineoperation, during which time in most systems, the main catalyticconverter has not attained light-off and is therefore, not active. As aresult, during cold-start large amounts of hydrocarbons may bedischarged into the atmosphere if additional measures are not taken. Theproblem is made worse by the fact that the engine requires rich fuel-airratio to operate during cold-start thus, increasing even further theamount of unburned hydrocarbons discharged. Therefore, to increase theeffectiveness of automotive emission control systems during cold start,and more importantly, the ULEV standards require that, the amount ofhydrocarbons discharged into the atmosphere during cold-start must bekept to extremely low levels.

Various schemes have been proposed for meeting the stringent ULEVstandards during cold start including through the use of electricallyheated catalysts (EHCs) to reduce the light-off time of the maincatalyst. Another suggested scheme includes the use of molecular sievestructures (hydrocarbon adsorbers) to adsorb and hold significantamounts of hydrocarbons until the converter has attained its light-offtemperature. Still, other schemes have been suggested involving acombination of electrically heated catalysts and adsorbers. Recently,improved in-line and by-pass exhaust control systems respectively havebeen disclosed in co-pending, co-assigned U.S. application Ser. No.08/234,680 and 08/259,459 (both herein incorporated by reference), usingbi-metallic valves to control exhaust gas flow during cold-start. In theformer, a hollow molecular sieve structure having a bi-metallic valve isused to achieve the ULEV standards. Co-pending, co-assigned U.S.application Ser. No. 08/284,356 (Guile), filed concurrently herewith andherein incorporated by reference, discloses a by-pass adsorber systemwherein flow patterns from a secondary air source are used to directexhaust gas flow to and away from the adsorber during cold-start.

There continues to be a need for, and accordingly, it is the object ofthe present invention, to provide an even simpler and more improvedengine exhaust systems capable of meeting the strict California ULEVstandards.

SUMMARY OF THE INVENTION

The invention relates to an engine exhaust system having a burn-offcatalyst located downstream from a hydrocarbon adsorber, in which theamount of hydrocarbons emitted during cold-start is significantlyreduced by use of a hydrocarbon adsorber constructed to reduce thelight-off time of the burn-off catalyst; this being accomplished withoutthe use of valves. Optionally, the exhaust system may further comprise amain catalytic converter or a three-way catalyst (TWC) disposed upstreamfrom the hydrocarbon adsorber.

In one significant aspect, the invention relates to a hydrocarbonadsorber (or molecular sieve structure) having (1) a first region whichforms an unobstructed or substantially unobstructed flow path forexhaust gases of an exhaust stream formed between an engine and aburn-off catalyst disposed downstream from the adsorber, and (2) asecond region abutting the first region, which forms a more restrictedflow path for the exhaust gases than the first region.

In a further aspect, the hydrocarbon adsorber of the invention includesa variable cell honeycomb structure in which the cells forming a firstregion are larger than the cells forming a second region to cause lessrestricted flow through the first region than through the second region.Alternatively, the first region occupies the central region of theadsorber, and the second region is the peripheral region surrounding thefirst region.

In one particular aspect, the invention relates to an engine exhaustsystem having a main catalyst having a light-off temperature; a housingdownstream of the main catalyst, the housing having an inlet and anoutlet end, and having disposed therein a molecular sieve structure orhydrocarbon adsorber for adsorbing hydrocarbons, the molecular sievestructure being characterized by a desorption temperature and having ahollow central core; a burn-off catalyst disposed downstream from theadsorber, the burn-off catalyst having a light-off temperature; anddiverting means disposed in the housing for passing secondary air to themolecular sieve structure to maintain the sieve temperature at a lowlevel until light-off of the burn-off catalyst has been achieved.Advantageously, the flow pattern of the secondary air is such as todirect a major portion of the exhaust gases of the exhaust streamthrough the central region of the adsorber after the main catalyst hasattained its light-off temperature.

In still another aspect, air diverters are disposed at the inlet end,outlet end, or both ends of the adsorber to control the exhaust gas flowthrough the central region of the adsorber.

Optionally, additional secondary air injection means can be disposed atthe outlet end of the adsorber or just before the burn-off catalyst toprovide additional oxygen needed to oxidize desorbed hydrocarbons or torestore stoichiometry as needed during vehicle operation for example.Also, the hydrocarbon adsorber may be catalyzed with catalysts capableof decomposing the NOx, CO and hydrocarbons in the exhaust stream toharmless components.

As used in this specification:

"molecular sieve" refers to crystalline substances or structures havingpore sizes suitable for adsorbing molecules. The term is generally usedto describe a class of materials that exhibit selective absorptionproperties. To be a molecular sieve, the material must separatecomponents of a mixture on the basis of molecular size and shapedifferences. Such materials include silicates, the metallosilicates,metalloaluminates, the AlPO₄ S, silico-and metalloaluminophosphates,zeolites and others described in R. Szostak, Molecular Sieves:Principles of Synthesis and Identification, pages 2-6 (Van NostrandReinhold Catalysis Series, 1989);

"zeolites" are crystalline aluminosilicates whose structures are basedon a theoretically limitless three-dimensional network of AlOx and SiOytetrahedra linked by the sharing of oxygen atoms, such as more fullydisclosed in U.S. Pat. No. 3,702,886, in British Specification No.1,334,243, published Oct. 17, 1973, in U.S. Pat. No. 3,709,979, and inU.S. Pat. No. 3,832,449, all of which are herein incorporated byreference;

"monolithic substrate" is any unitary body or substrate formed from, orincorporating molecular sieve material; as used herein, a honeycombsubstrate is a form of a monolithic substrate, but a monolithicsubstrate is not necessarily a honeycomb substrate; "light-offtemperature" of a converter is the temperature at which a catalyticconverter can convert 50% of carbon monoxide or hydrocarbons or NOx;

"light-off time" of a catalytic converter is the amount of time requiredto attain light-off temperature;

"fluidics" is used herein to describe the mechanism or process ofdiverting exhaust gas flow either through or away from the centralregion of the molecular sieve structure using a smaller stream of fluid;and

for ease of discussion, the terms "adsorber" and "adsorption" as usedherein are intended to encompass both adsorption and absorption as theseterms are generally known to persons skilled in the art and as definedin Webster's Ninth New Collegiate Dictionary (1985); it is contemplatedthat both processes of adsorption and absorption occur in the molecularsieve structure of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the flow profile across an oval extrudedhoneycomb substrate in an oval can at various space velocities;

FIG. 1a is a similar graph illustrating the flow profile across anadsorber disposed in a non-symmetrical can design such as shown in FIG.2a below, but in which the adsorber is a uniform extruded honeycombstructure having no hole therethrough;

FIG. 2 is a sectional (longitudinal) view of one aspect of the inventionshowing an exhaust system in which exhaust gas flows from the engine toa main catalyst, thence to the adsorber of the invention having an openflow channel therethrough, and thence to a burn-off catalyst, FIG. 2a isa cross-sectional view of yet another aspect of the hydrocarbon adsorbersystem of the invention showing a non-symmetrical can design and anadsorber construction in which the hole or region of less obstructedflow is positioned along the exhaust path of least resistance;

FIG. 3 is a cross-sectional view of another embodiment of thehydrocarbon adsorber of the invention having large central cells andsmaller peripheral cells;

FIG. 4 is a graph showing the flow profile across a similar ovalhoneycomb substrate but one having a 0.50" hole cut through its center;

FIG. 5 is a graph showing the flow profile through a similar ovalhoneycomb as in FIG. 4, but one having a 0.75" hole cut through itscenter;

FIG. 6 is a graph showing the burnoff catalyst inlet temperature as afuinction of time and vehicle speed for the system of FIG. 2;

FIG. 7 is a graph showing the hydrocarbon emissions on both the inletand the outlet ends of the adsorber on FIG. 2 as a function of time andvehicle speed;

FIG. 8 is a graph comparing the adsorption efficiency of the adsorber ofthe invention with the adsorption efficiency of a standard adsorberhaving no open flow region;

FIGS. 9a to 9e illustrate another aspect of the invention using fluidicsor flow dynamics as defined above to direct flow through and away fromthe open central core region of the adsorber;

FIGS. 9a and 9b show the use of a cone-shaped flow diverter disposed onthe inlet and outlet ends of the adsorber respectively, to direct flowaway from the central hole during cold start;

FIGS. 9c and 9d show the optional use of an additional flow diverter atthe inlet end of the adsorber to direct air through the hole afterlight-off and after the first diverter at the outlet end of the adsorberhas been deactivated;

FIG. 9e shows the use of a air injection tube possessing a diverterplate to direct flow away from the central hole during cold start;

FIG. 10 is a sectional (longitudinal) view of the system of FIG. 9dshowing the direction of exhaust flow before the burn-off catalyst hasattained light-off;

FIG. 11 is a sectional (longitudinal) view of another embodiment of theinvention using as adsorbers, two molecular sieve structures placed inseries in the housing, each having a different or similar hydrocarbonselectivity;

FIG. 12 is a graph showing the exhaust gas velocity measured in thecenter of the hole on the outlet end of the adsorber as a function ofthe volumetric flow rate of the secondary air of the diverter beforelight-off; and,

FIG. 13 is an enlarged illustration of the air injection tube/diverterplate configuration depicted in FIG. 9e.

REFERENCE NUMERALS IN THE DRAWINGS

10, 30 . . . adsorber;

15, 32 . . . open flow region;

20 . . . honeycomb adsorber;

22 . . . cells;

25 . . . central region having large cells;

28, 50 . . . peripheral region having normal/smaller cells;

35 . . . a heating device such as an electrically heated honeycombstructure, or a main catalytic converter such as a three-way catalyst, alight-off catalyst or an electrically heated catalyst, close-coupledwith the engine;

40 . . . burn-off catalyst;

45 . . . circular air injection port;

47 . . . multiple cone-shaped, directional nozzles;

53 . . . secondary air injection collar;

55 . . . nozzles;

57 . . . air injection port;

60 . . . single cone-shaped nozzle;

65 . . . cone-shaped secondary air stream;

70 . . . ZSM-5-coated cordierite adsorber; and

75 . . . USY-zeolite-coated cordierite adsorber.

80 . . . diverter plate

81 . . . diverter plate support system

82 . . . diverter plate support

83 . . . diverter plate threaded post

85 . . . radially directed secondary air stream

DETAILED DESCRIPTION OF THE INVENTION

The object of the invention, that is, to reduce the light-off time ofthe burn-off catalyst sufficiently to meet the stringent ULEV standardsis achieved by taking advantage of the flow dynamics of the exhauststream through a monolithic adsorber; light-off is achieved before theadsorber heats up and begins to desorb any trapped hydrocarbons. We haveobserved that the flow dynamics of the engine exhaust system of theinvention is such that the exhaust gases in the exhaust gas streambetween an engine and a burn-off catalyst tend to flow through a path ofleast resistance from the engine and out to the atmosphere. In thestandard cone-shaped exhaust can design in which a generally circularhoneycomb adsorber is centrally disposed in the can, this path passesthrough the central region of the adsorber. As a result, the exhaustgases tend to flow through the central region of the cellular adsorberat a faster rate than through the peripheral regions of the structures.For a non-symmetrical can design, the same flow dynamics is observed,that is the exhaust gases tend to flow faster through the path of leastresistance to flow. Hereinafter, this flow path of least resistance willbe referred to as "the exhaust path."

This observed flow dynamics is illustrated in FIG. 1 for an ovalextruded honeycomb substrate centrally disposed in an oval can design.The flow rates through various sections of the substrate were measuredusing six probes whose relative positions across the substratecross-section are shown in the inset. To determine the flow profileacross the substrate cross-section, the substrate was contacted with airat space velocities ranging from relatively low (30 cubic feet perminute (CFPM), line a) to high (150 CFPM, line f) space velocities. Asshown in FIG. 1, at the lower velocities, the air flow across thesubstrate cross-section as measured by probes 1 through 6, is fairlyuniform. The position of the probes across the substrate cross-sectionis indicated by the numbers 1-6 in the insets. As the space velocityincreases, the flow profile becomes increasingly variable, with the flowrate at and near the center of the substrate being higher than the flowrates through the peripheral regions. Similarly, for an adsorberdisposed in a non-symmetrical can design such as shown in FIG. 2a, thesame tendency is observed. The flow profile of the can design of FIG. 2afor a honeycomb adsorber having a uniform cross-section (that is,without an open flow region such as a hole or larger cells) is shown inFIG. 1a. Again, flow through the region of the adsorber found along theflow path of least resistance (regions 4-6), is faster than the flowrate through the peripheral region (marked 1-3, i.e., the region awayfrom the path of least resistant flow).

We have found that by taking advantage of the above-described flowdynamics, the light-off time of the burn-off catalyst can besignificantly reduced. In particular, for the cone can design forexample, we have discovered that by leaving the central region of thesubstrate open or substantially less obstructed than the peripheralregions, the flow dynamics observed in the extruded honeycomb adsorber(FIG. 1), can be enhanced.

In one particularly useful embodiment (FIG. 2), the hydrocarbon adsorber10 is formed with a hole or open flow region 15 in the center asillustrated in the figure. In this embodiment, a portion of the exhaustgas stream is allowed to flow through the center of the adsorberdirectly to the burn-off catalyst to quickly bring the burn-off catalystto its light-off temperature in a significantly shorter period of timethan would be required using a regular honeycomb adsorber with no hole.Preferably, the burn-off catalyst attains its light-off temperaturebefore the adsorber attains its desorption temperature to avoidpremature desorption which will result if the timing were reversed. Theamount of exhaust needed to achieve this goal will vary for each systemand must be determined by experimentation. However, the amount should besufficient to bring the burn-off catalyst to its light-off temperature,but not so large as to force adsorbed hydrocarbons to escape from theadsorber. For the cone can design used in the experiment, less thanapproximately of the exhaust gas volume was sufficient to bring theburnoff catalyst to its light-off temperature in a very short period oftime and before the adsorber had reached its desorption temperature. Forsystems where an electrically heated catalyst or a heating device isclose coupled with the engine, the amount of exhaust gases that wouldrequired to bring the burn-off catalyst to its light-off temperature maybe as low as 5-20% depending on the particular exhaust systemconfiguration.

Similar results can be obtained by constructing the adsorber in anyshape wherein there is less obstructed flow through the exhaust path ofleast resistance. For the cone can design in which a generally circularadsorber is centrally disposed in a cone-shaped can such as in FIG. 2preferably, the region of less obstructed flow 15 is along the centralregion of the adsorber so that flow through the center is lessrestricted than through the peripheral region. In addition to theconstruction having a hole through the center, the adsorber can also bea honeycomb structure, preferably an extruded honeycomb structure 20 inwhich the cells 22 forming the central region 25 are larger than thoseforming the peripheral region 28 as illustrated in FIG. 3. For anon-symmetrical can design (FIG. 2a), the adsorber 30 can be constructedwith a region of less restricted flow 32 having a hole or larger cells,displaced from the center as illustrated in FIG. 2A; alternatively, thehole can be cut into the edge of the honeycomb structure along itslength. Here, the exhaust flow path of least resistance is along anexhaust stream connecting the engine (optionally through a light-offcatalyst 35) to the burn-off catalyst 40. Therefore, the unobstructedflow region (hole or larger cells) is positioned along this path,preferably in the region marked 4-6 in FIG. 2a.

This flow dynamics is further illustrated in FIGS. 4 and 5, in which a0.5" and 0.75" hole respectively, have been cut out of the center of twooval extruded honeycomb structures in oval cans. As shown in thefigures, the flow rate through the central region of the substrates issignificantly higher than observed in the regular substrate with no holein the middle (FIG. 1). Further, it is observed that the flow ratethrough the central region of the adsorber is higher for the monolithhaving a 0.75" size hole in its center than for the monolith having a0.5" hole. This variation in flow rate becomes more pronounced as thespace velocity increases from 30 CFPM (line a) to 150 CFPM (line f) inboth cases, as was observed in FIG. 1.

In the exhaust system of the invention, the burn-off catalyst comes toits light-off temperature quicker because the portion of the exhaustflowing through the central hole, avoids contact with the adsorber whichhas a certain mass and which therefore acts as a heat sink. Whereas, theunrestricted flow through the hole causes the heat in the exhaust streamto be transferred directly to the burn-off catalyst downstream from theadsorber. At the same time, since a substantial portion of the exhaustgases flow through the hole, a small portion of the exhaust gas streampasses over the adsorber where hydrocarbons are adsorbed and held untilthe adsorber attains its desorption temperature. We have found that byusing the construction described herein, the burn-off catalyst reachesits light-off temperature before the adsorber attains its desorptiontemperature, thus avoiding premature desorption which has been a problemwith some existing adsorption systems. The time required to bring theburn-off catalyst to its light-off temperature can be further reduced byplacing a main catalytic converter having a light-off temperature,upstream from the adsorber, and downstream from the engine. Examples ofuseful main catalytic converters for this application include anythree-way catalyst, a light-off catalyst, an oxidation catalyst, anelectrically heated catalyst and the like. Alternatively, instead of amain catalyst, a heating device such as a heated honeycomb structure oran electrically heated catalyst can be disposed in the same position,close coupled to the engine to heat up the exhaust gas stream. Due toits closer proximity to the engine, the main catalytic converter attainsits light-off temperature sooner than does the burn-off catalyst whichis further downstream. The adsorber system of the invention allows heatfrom the main catalyst to reach the burn-off catalyst significantlyfaster than with conventional adsorber systems. As a result, theburn-off catalyst in the exhaust system of the invention, attains itslight-off temperature at a faster rate than observed with other systems.

This and other advantages of the adsorber of the invention areillustrated in FIGS. 6-8. FIG. 6 compares the inlet temperature of theburn-off catalyst when used in conjunction with an adsorber having a0.5" central hole, with the burn-off inlet temperature when placeddownstream of a standard cellular adsorber without the central opening.As shown in the diagram, the inlet temperature of the burn-off catalystis higher for the adsorber of the invention (i.e., with the hole), thanit is for a standard adsorber without the hole. In fact, in the designof the invention, the temperature of the burn-off catalyst increasessteadily and rapidly from engine start-up, and the burn-off catalystattains light-off significantly sooner than the standard design. Forexample, after only about 90 seconds from engine start-up, the inlettemperature of the burn-off catalyst using the adsorber of theinvention, had reached about 200° C. On the other hand, in the standardadsorber (that is, without the central hole), the burn-off inlettemperature does not reach 200° C. until more than 150 seconds hadelapsed from engine start-up.

As shown in FIG. 7, the presence of the hole in the central region ofthe adsorber has little or no adverse effect on the adsorptionefficiency as determined by comparing the hydrocarbon emission at theengine, with the emission at the outlet end of the adsorber. Asignificant amount of the hydrocarbon emitted from the engine isadsorbed by the adsorber. Similarly, it is believed that the presence ofthe hole does not result in any significant loss of adsorptionefficiency. This is illustrated by FIG. 8 which compares the hydrocarbonemission at the outlet end of a cellular adsorber having a 0.5" hole inits center, with a standard cellular adsorber. As seen in the diagram,there is little noticeable difference between the adsorption efficiencyof the adsorbers.

Once the light-off catalyst has attained its light-off temperature itsoon attains its full operating temperature and is then capable ofconverting the NOx, hydrocarbon, and carbon monoxide in the exhauststream. Heated exhaust gases passing through the now hot light-offcatalysts soon brings the burn-off catalyst to its light-offtemperature. As stated above, we have observed that in the presentdesign, the adsorber reaches its desorption temperature only after theburn-off catalyst has attained its full operating temperature. Once theadsorber reaches its desorption temperature, any trapped hydrocarbonsare desorbed or released from the adsorber into the exhaust stream andthence to the burn-off catalyst where the hydrocarbons are converted toinnocuous products and discharged into the atmosphere. Preferably, afterthe burn-off has attained light-off, a portion of the exhaust gascontinues to flow through the peripheral region of the adsorber to aidin the desorption of hydrocarbons.

We have found that air flow through the central hole region can becontrolled using flow diverters which may be placed on the inlet oroutlet end of the adsorber as shown in FIGS. 9a to 9e. Relying on fluiddynamics (fluidics), during cold start the flow diverters are used todivert exhaust gases away from the open flow central region (or hole),and towards the peripheral regions of the adsorber. This is achieved byprojecting a jet of secondary air into the exhaust gas flow path todivert the exhaust gas flow in a desired direction. The spray patternfrom each of the nozzles is further illustrated in the diagrams.

During cold-start the flow diverter is placed on the inlet end of theadsorber housing as shown in FIG. 9a to direct exhaust gas flow awayfrom the hole and into the adsorber as shown. Alternatively, the flowdiverter can be placed at the outlet end (downstream) of the adsorber asshown in FIG. 9b, to redirect exhaust gases through the adsorber. Forcertain applications where the initial speed during cold start isconsiderably high, or where the engine exhaust is particularly high inhydrocarbons, flow diverters may be placed both at the inlet and outletend locations to ensure that a substantial amount of the exhaust gasstream is directed through the adsorber during cold start. When thelight-off catalyst has attained its light-off temperature, the flowdiverter is deactivated, thus allowing the exhaust gases which by noware relatively hot, to flow to the burn-off catalyst through the opencore central region and thereby quickly bring the burn-off catalyst toits light-off temperature. After the burn-off catalyst has attained itslight-off temperature, some portion of the exhaust gases continue toflow through the adsorber to aid with desorption.

During cold start, the flow diverters operate by projecting at highvelocity, a small jet of secondary air stream from a secondary airsource, into the engine exhaust stream to divert the exhaust gas flowaway from the central hole, and towards the peripheral region of theadsorber.

The diverter can be in various forms. Preferably, the diverting means issuch as to present as little obstruction to exhaust flow as possiblewhen no secondary air pattern is present. For example, in one embodiment(FIG. 9a), the flow diverting means is a thin tubular air injection port57, running parallel to the exhaust gas flow and fitted with a smallcone-shaped nozzle 60, positioned at the inlet end of the adsorberhousing, through which secondary air in the form of a cone-shaped jet 65can be injected into the housing to effectively block off exhaust gaspassage through the hole as shown. The nozzle 60 is capable ofprojecting a cone-shaped air stream 65 from a secondary air source, todivert the exhaust gases towards the peripheral region 50 of theadsorber and away from the central hole 15 during cold start by blockingpassage of exhaust gases through the hole. The nozzle 60 can consist ofany directional air outlet capable of directing secondary air stream toeffectively form a shield to prevent exhaust gas flow through the regionof the adsorber along the exhaust path during cold start. In theembodiment illustrated, during cold start, the flow diverting means isactivated by allowing secondary air to pass through the air injectionport 57 to the nozzle 60. The nozzle 60 is constructed such that thesecondary air forms an umbrella-like shield in front of the hole orcentral region of the adsorber thereby diverting flow away from the hole15 and into the peripheral regions 50 of the adsorber.

Alternatively, the air injection port 57 and the cone-shaped nozzle 60can be positioned at the outlet end of the adsorber to redirect exhaustgases back through the hole during cold start as shown in FIG. 9b.

In another embodiment (FIG. 9c), the diverter is a circular, tubular airinjection port 45 fitted with a plurality of preferably smallcone-shaped, directional nozzles 47 as shown. In this embodiment, afterthe light-off catalyst has attained its light-off temperature, thecone-shaped flow diverter 60 is deactivated and secondary air jets 65are allowed to project from nozzles 47 of air injection port 45 todeflect or divert a substantial portion of the engine exhaust streamtoward the hole 15 and away from the peripheral regions 50 of theadsorber as indicated.

Similarly, secondary air jets for diverting flow after light-off may beintroduced into the housing through an air injection collar 53 having aplurality of cone-shaped directional nozzles 55 as shown in FIGS. 9d and10. As with the circular air injection port of FIG. 9c, here again,before light-off, flow through the center region is prevented by use ofa flow diverter positioned at the outlet end of the adsorber as shown inFIG. 10, to redirect exhaust gas flow back through the center andthrough the peripheral region of the adsorber. Alternatively, the airinjection port 57 may be further elongated to extend from the outlet tothe inlet end of the adsorber such that the nozzle 60 is disposed infront of the hole 15 on the inlet end of the adsorber.

After light-off, flow through nozzle 60 is discontinued and secondaryair flow through the nozzles 55, are used to effectively force a largeportion of the engine exhaust gases through the hole 15 as shown inFIGS. 9c and 9d to quickly bring the burn-off catalyst to its effectivelight-off temperature. While the tubular air injection port and the airinjection collar are shown on the inlet end of the housing, such devicesmay also or instead be located on the outlet end of the adsorber housingto redirect air away from the hole in the same manner described above.

In a another embodiment, as depicted in FIG. 9e, the diverter comprisesthe thin tubular air injection port 57, running parallel to the exhaustgas flow and fitted with a diverter plate 80, positioned at the inletend of the adsorber housing, through which secondary air in the form ofa radially directed jet 85 can be injected into the housing toeffectively block off exhaust gas passage through the hole as shown.Preferably, the direction of the jet is approximately perpendicular tothe direction of the exhaust gas flow. The air injection port 57 coupledwith the diverter plate 80 are capable of projecting an air stream 85from a secondary air source, to divert the exhaust gases towards theperipheral region 50 of the adsorber and away from the central hole 15during cold start by blocking passage of exhaust gases through the hole.In other words, the air injection port 57 coupled with the diverterplate 80 combine together to direct a secondary air stream toeffectively form a shield to prevent exhaust gas flow through the regionof the adsorber along the exhaust path during cold start. In theembodiment illustrated, during cold start, the flow diverting means isactivated by allowing secondary air to pass through the air injectionport 57 to the diverter plate 80. The diverter plate 80 diverts thesecondary air forming a radially directed air shield in front of thehole or central region of the adsorber thereby diverting flow away fromthe hole 15 and into the peripheral regions 50 of the adsorber.

FIG. 13 illustrates, in more detail, the flow diverter means used in thepreferred embodiment illustrated above in FIG. 9e. The diverter plate 80is positioned, a variable slot distance W, in front of the outlet of theair injection port 57, through the use of a diverter plate supportsystem 82. Diverter plate support system 81 consists of a support memberwhich is secured within the inside circumference of the tubularinjection port and a threaded post 83 which extends out of the airinjection port 57. Diverter plate 80 is directly attached to threadedpost 83 allowing for the slot width to be varied. It should be notedthat, although the preferred slot width is that width which results inan air flow which is perpendicular to the direction of the exhaust gas,the slot width may be increased resulting in a radial flow which is morecone-like in shape.

Depending on the particular application, the engine exhaust system ofthe invention can be constructed with any one or a combination of airflow diverters. For example, an exhaust system can comprise a tubularair injection port having a plurality of cone-shaped directionalnozzles, an air injection collar having a plurality of nozzles, atubular air injection port possessing a cone-shaped air injectionnozzle, a tubular air injection port possessing a diverter plate, an airinjection tube, an air "knife" and/or combinations of these. Similarly,an exhaust system of the invention can comprise diverters at the inletend, the outlet end, or on both ends of the adsorber housing.

Preferably, the flow of the secondary air source is at high velocityimmediately after engine start-up in order to sufficiently direct all orsubstantially all of the engine exhaust gases away from the hole. Duringcold-start, secondary air flow through the diverter, though at lowvolume is able to achieve significant jet (strength) due to its highvelocity. As the engine speed increases, and as the exhaust gastemperature begins to increase, the velocity of the secondary air jetmay be gradually reduced to allow more exhaust gas to flow through thehole in order to heat up the burn-off catalyst. After the burn-offcatalyst has reached its light-off temperature, flow of secondary airmay be terminated to allow substantially free and unobstructed flow ofthe exhaust gases through the hole. Preferably, the size of the diverteris small compared to the size of the housing and the exhaust pipe sothat after the secondary air source is discontinued exhaust gas flowthrough the hole is not significantly affected by the presence of thediverter.

As described above, after light-off secondary air flow is desirable toeffect combustion of desorbed hydrocarbons. Additional air needed foroxidation may be provided through an auxiliary secondary air sourcewhich is preferably placed at the outlet end of the housing justupstream from the burn-off catalyst. Alternatively, after light-off, andafter the flow diverter has been deactivated, secondary air may bepassed into the adsorber and housing through a plurality of nozzlesformed around a circular air injection port or preferably through an airinjection collar similar to that shown in FIGS. 9c through 10. After theadsorber is fully desorbed of hydrocarbons, the secondary air patternmay be discontinued. This flow of secondary air into the adsorber alsoprovides the added advantage of diverting the exhaust gases away fromthe adsorber and through the hole as described above.

Secondary air from the same or a separate source may also be used asneeded during vehicle operation to control the adsorber temperature. Forexample, after the engine has heated up, secondary air can be injectedinto the adsorber in the same manner described above, to provide coolingair to the adsorber after the engine has heated. This additionalsecondary air will operate to maintain the adsorber at temperaturesbelow its desorption temperature in the period prior to the burn-offcatalyst attaining its light-off temperature to prevent prematuredesorption. In addition, secondary air may be introduced into thehousing at any time as needed for example, to restore stoichiometry, orin certain applications, to provide additional air that may be necessaryfor oxidation.

Although nonlinear, we have found that the larger the hole size, thegreater the proportion of exhaust gases that flow through the hole andthe central region of the adsorber. Preferably, the hole size or thesize of the large cells in the central region of the adsorber are suchas to allow a sufficient amount of the exhaust gases to flow through theadsorber during cold start to meet the Federal Test Procedure (FTP)standards. At the same time, the hole size should be such as to allow asufficient amount of exhaust gases to reach the burn-off catalyst so asto quickly bring it to its light-off temperature. The optimal hole sizefor a given application may be determined experimentally and will dependon such factors as the engine size, the volume of the adsorber, thegeometric surface area of the adsorber, the efficiency of the particularadsorber material, the operating conditions and other variables.

In the following examples, a round cordierite extruded honeycombsubstrate having an outside diameter of 4.66 inches, and a frontal areaof 17.05 square inches, was cut into six samples each measuring 4 inchesin length. Three of the samples were washcoated with ZSM-5, a zeoliteknown to adsorb low molecular weight hydrocarbons. The remaining threesamples were washcoated with ultra stable Y (USY), a zeolite known tohave good adsorption capacity for higher molecular weight hydrocarbons.One of the ZSM-5 coated samples was drilled with a hole through itscenter measuring 0.5" and another with a hole measuring 0.75" runninglongitudinally between the two end faces of the substrate and parallelto the cells. Similarly, one of the USY coated samples was drilled witha hole measuring 0.5" and another with a hole measuring 0.75". Thecontrol adsorber consisted of one each of the ZSM-5 and USY coatedsamples without holes.

In the first experiment, the ZSM-5 coated adsorber sample 70 having a0.5" hole was placed in the adsorber housing, in series with theUSY-coated sample 75 also having a 0.5" hole, with the holes lined up asshown in FIG. 11. Similar experiments were staged using the 0.75" holesamples and the control samples having no holes. Using a 3.8 literengine, the samples were tested using the FTP test. No flow diverterswere used in these experiments. During cold-start the amount ofhydrocarbons passing through the control adsorber (no holes) asdetermined by the hydrocarbon emission at the outlet end of the adsorberhousing, was in the range of 60-65%. In other words, the control resultsindicate that some 35-40% of the hydrocarbons emitted from the engineduring cold start were not adsorbed., i.e., the hydrocarbons passedthrough the open cells of the honeycomb adsorber or were otherwise notpicked up by the adsorber. For the set of adsorbers having the 0.5"holes the amount of hydrocarbons passing through the adsorber, andtherefore actually adsorbed, was in the range of 50-55%. And for theadsorber having a 0.75" hole, the amount of hydrocarbons passing throughthe adsorber, and therefore adsorbed, was in the range of 35-40%.

By forming the 0.5" and 0.75" holes which represent 1.15% and 2.58% ofthe adsorber frontal area respectively, the amount of resultingunadsorbed hydrocarbons increased by 10% and 25% respectively. At thesame time however, the burn-off catalyst attained light-offsignificantly quicker with the holes because heated exhaust gases fromthe main catalyst reached the burn-off catalyst in a shorter period oftime than without the holes. Accordingly, it is contemplated that evenwithout the use of diverters, adsorbers having holes of sizes rangingfrom about 0.5% up to about 50% of the adsorber frontal area will beuseful for the practice of the invention. As described above thepreferred adsorber frontal area hole size area should be empiricallydetermined for each system designed taking into account theaforementioned factors such as engine size, the volume of the adsorberet al. With diverters, much larger hole sizes can be used since thediverter can be used to ensure that exhaust gases do not pass throughthe hole before light-off

To illustrate the effectiveness of the fluidic flow diverter of theinvention as measured by the degree to which hydrocarbon-bearing exhaustgas is successfully prevented from passing through the hole during coldstart, the following experiment was done. With the diverter activated ina configuration similar to those illustrated in FIGS. 9a, 9b and 10,exhaust gas was simulated by passing air into the adsorber housing at avolumetric flow rate of 40 cubic feet per minute (cfpm). An elongatedflow diverter was placed in the housing along the center of the hole,extending from the outlet to the outlet end of the adsorber such thatthe cone-shaped nozzle is disposed in front of the hole at the inlet endof the adsorber. With the simulated exhaust gas flowing at the specifiedrate, the flow diverter was activated by passing secondary air to form acone-shaped air shield over the entrance of the hole. Using a probepositioned in the region of the hole at the outlet end of the adsorber,the linear velocity of the exhaust gas (simulated) passing through thehole was measured as a function of the secondary air volumetric flowrate given in the graph in cubic feet per minute (cfpm). The results areplotted in FIG. 12. As shown, as the secondary air flow rate increases,the exhaust gas is increasingly diverted to the peripheral regions ofthe adsorber as indicated by the decrease in the exhaust gas flow ratethrough the hole as measured by the probe. The higher the secondary airflow rate, the stronger the air jet, and the more the amount of exhaustgas diverted. Specifically, the flow rate of the exhaust gas measured atthe outlet of the hole falls from a maximum or about 2300 linear feetper minute with no diverter, to about 300 fpm at a secondary air flow ofabout 3 cfpm.

Even though the adsorber used in the above examples consisted of twocordierite monoliths washcoated with different molecular sieve materialsand used in combination as described, other constructions and assembliesare possible and will be clear to those skilled in the art. For example,instead of washcoating different monoliths with different zeolites, theadsorber can also consist of one monolith washcoated with one or amixture of different molecular sieve materials (in this case, zeolites).Also, instead of washcoating a substrate with the zeolites, the adsorbercan consist of an extruded zeolite formed in the manner describedhereinafter. Without intending to be bound by theory, it is postulatedthat the washcoated cordierite specie of adsorber may adsorb morehydrocarbons because it tends to stay cooler for a longer period oftime, thereby avoiding premature desorption which results when theadsorber reaches its desorption temperature before the burn-off catalystattains light-off. This tendency to remain cooler for a longer period oftime is believed to be aided by the presence of the much densercordierite substrate which acts as a heat sink thus absorbing some ofthe heat. In contrast, when the adsorber consisted of an extrudedzeolite (which is less dense than cordierite), the cooling effect wasreduced. To achieve the same cooling effect with extruded zeoliteadsorbers, in another experiment a cordierite mass (such as an extrudedcordierite body) was placed in the housing, upstream from the extrudedzeolite. The same cooling effect observed with washcoated cordieritesubstrates was again observed. It is expected that any similar structurecapable of acting as a heat sink can be used to achieve the same resultwhere premature desorption may be a problem.

By using flow diverters as described above, adsorbers having larger holesizes or more open central regions can be used since the diverter can beused to direct substantially all of the exhaust gases away from thecentral region and into the peripheral regions. In this embodiment usingflow diverters, the hole size may be as high as 50% of the adsorberfrontal area or more provided that the remaining adsorber is capable ofadsorbing a sufficient amount of the hydrocarbons during cold start tomeet the FTP test standards.

In one particularly useful embodiment, during cold start, substantiallyall of the exhaust gases entering the adsorber housing are diverted tothe adsorber and away from the hole using flow diverters disposed eitherin the inlet end of the adsorber housing, the outlet end, or on bothends of the housing. In this embodiment, potentially all of the exhaustgases entering the housing are forced to pass through the adsorber untilthe main catalyst attains its light-off temperature. Once the maincatalyst reaches its light-off temperature, the amount of hydrocarbonscontained in the exhaust stream entering the adsorber housing becomessufficiently low. Therefore, as soon as the main catalyst attainslight-off, the diverters are adjusted or deactivated to allow themaximum amount of exhaust gases to pass through the hole and therebybring the burn-off catalyst to its light-off temperature.

Useful molecular sieve materials for the invention include silicates(such as the metallosilicates and titanosilicates) of varyingsilica-alumina ratios, metalloaluminates (such as germaniumaluminates),metallophosphates, aluminophosphates (such as silico-andmetalloaluminophosphates (MeAPO), SAPO, MeAPSO), gallogerminates andcombinations of these. Examples of useful metallosilicates includezeolites, gallosilicates, chromosilicates, borosilicates,ferrisilicates. Examples of zeolites which are particularly useful forthe invention include, ZSM-5, Beta, gmelinite, mazzite, offretite,ZSM-12, ZSM-18, Berryllophosphate-H, boggsite, SAPO-40, SAPO-41, andcombinations of these, most preferably, ZSM-5, Beta, Ultra-stable Y(USY), and mordenite. For such applications, zeolites having highsilica/alumina ratios (greater than 10), are more thermally stable andare therefore preferred. Furthermore, it is contemplated thatapplications maintained under reducing conditions, activated carbon maybe the material of choice.

It is well known that during cold start, molecular sieve zeolites notonly trap hydrocarbons but also cause cracking of some hydrocarbons(i.e., coking). To prevent coking, the adsorber may be catalyzed withsuitable catalysts. As is well known in the art, noble metal oxidationcatalysts such as platinum, rhodium, and palladium, may be added tozeolite molecular sieve to ensure oxidation of the carbonaceousmaterials which may result from coking. Any catalyst capable ofconverting hydrocarbons to water and carbon dioxide may be added to thezeolite. Such catalysts are well known in the art. For example, noblemetal catalysts, such as platinum, rhodium, palladium, and mixtures ofthese are widely used in automotive catalytic converters. Thesecatalysts are capable not only of oxidizing hydrocarbons but also ofconverting carbon monoxide and NOx in the engine exhaust stream toinnocuous products. Such catalysts may be incorporated into the adsorberor molecular sieve structure by known methods. It is also known thatcertain zeolite/noble metal combinations such as disclosed inco-assigned U.S. Pat. No. 5,244,852 (herein incorporated by reference)function as three-way catalysts to convert.

As discussed above, three-way converters which additionally convert NOxand carbon monoxide to non-toxic by-products may also be used in thepractice of the invention. Typically, three-way catalysts used inautomotive applications comprise noble metals such as platinum and/orpalladium, and rhodium. Examples of such catalysts includeplatinum/palladium/rhodium on gamma alumina with rare earth oxides(e.g., ceria), and platinum on ceria-alumina combined with rhodium onzirconia.

The hydrocarbon trap or molecular sieve structure of the invention, maybe utilized in any number of forms. For example, the molecular sieve orzeolite may be utilized directly in the form of beads or pellet, or itmay be embedded in, or coated on porous substrates. The molecular sievematerial can be applied onto the substrate by any known method such asfor example, by conventional washcoat or spraying techniques. In thewashcoat technique, the substrate is contacted with a slurry containingthe molecular sieve and other components such as temporary binders,permanent binders or precursors, dispersants and other additives asneeded. Such methods are well known in the art. The permanent binder inthe slurry includes for example, aluminum oxide and its precursors,silica, titania, zirconia, rare earth oxides, and their precursors,spinel and precursors. The molecular sieve slurry is then applied (forexample, by repeated spraying or dipping) to the substrate until thedesired amount of molecular sieve material has been applied. One usefulmethod for forming zeolite on the surface of a substrate is disclosed inU.S. Pat. No. 3,730,910, herein incorporated by reference.

In one particularly useful embodiment, the molecular sieve is zeolite inthe form of a porous monolithic structure formed by extruding thezeolite into a honeycomb structure U.S. Pat. No. 4,381,255, hereinincorporated by reference, discloses a process for producing binderlesszeolite extrudates by extruding a mixture containing equal amounts of azeolite powder, a metakaolin clay and a near stoichiometric causticsolution, in which the clay in the extrudate crystallizes to form acoherent particle that is essentially all zeolite. Similarly, U.S. Pat.No. 4,637,995, herein incorporated by reference, discloses a method forpreparing a monolithic zeolite support comprising a ceramic matrixhaving zeolite dispersed therein.

Another useful method of forming the molecular sieve structure includesembedding or coating zeolite on a metal, metal alloy, ceramic, or glassceramic substrate, such as extruded honeycomb substrates, as disclosedin U.S. Pat. No. 4,657,880 herein incorporated by reference.

The adsorber can also be formed by in situ growth of zeolite, that is,by crystallizing zeolite on the surface of a metal, metal alloy,ceramic, or glass ceramic substrate. A method for crystallizingstrong-bound zeolites on the surfaces of monolithic ceramic substratesis disclosed in U.S. Pat. No. 4,800,187, herein incorporated byreference.

The substrate can be any material suitable for high temperatureapplication such as certain metals, metal alloys, ceramics,glass-ceramics, glass, high surface area-high temperature stable oxides,and combinations of these materials. Examples of useful substratematerials include, cordierite, mullite, clay, talc, zircon, zirconia,spinel, alumina, silica, borides, lithium aluminosilicates, aluminasilica, feldspar, titania, fused silica, nitrides, carbides and mixturesof these. Useful metals for the substrate include, substrates formed ofiron group metals such as Fe--Al, Fe--Cr--Al alloys, stainless steel,and Fe--Nickel alloys.

U.S. Pat. No. 4,631,267, herein incorporated by reference, discloses amethod for producing a monolithic support structure for zeolite by (a)mixing into a substantially homogeneous body (i) a zeolite, (ii) aprecursor of a permanent binder for the zeolite selected from the groupconsisting of alumina precursors, silica precursors, titania precursors,zirconia precursors and mixtures of these, and (iii) a temporary binder;and extruding the mixture to form a porous monolithic molecular sievestructure.

In addition to the embodiments discussed above, it will be clear topersons skilled in the art that numerous modifications and changes canbe made to the above invention without departing from its intendedspirit and scope.

We claim:
 1. A method of treating a hydrocarbon-containing engineexhaust stream comprising: causing exhaust gases from an engine to flowthrough a main catalytic converter having a light-off temperature andthereafter through a molecular sieve structure and thereafter through aburn-off catalyst structure having a light-off temperature therebyforming an exhaust gas stream, wherein the molecular sieve structure hasan inlet and outlet end disposed in a housing, a desorption temperature,and a first substantially unobstructed flow region, and a second moreobstructed flow region abutting the first region, the first region beingdisposed in the exhaust stream such that the first region provides asubstantially unobstructed flow path for the exhaust gases, the methodfurther including, prior to the main catalytic converter attaining itslight-off temperature, activating a secondary air source for diverting asubstantial portion the exhaust gases away from said first region andtoward said second region of the molecular sieve.
 2. The method oftreating a hydrocarbon-containing engine exhaust stream as claimed inclaim 1 wherein, after the main catalytic converter has attained itslight-off temperature, deactivating the secondary air to cause asubstantial portion of the exhaust gases to flow back towards said firstregion and away from the second region.
 3. The method of treating ahydrocarbon-containing engine exhaust stream as claimed in claim 2wherein after the burn-off catalyst has reached its light-offtemperature, activating the secondary air to cause at least a portion ofthe exhaust gases to flow into the second region of the molecular sievestructure to thereby desorb hydrocarbons therefrom.
 4. The method ofclaim 3 wherein the desorbing involves further contacting the molecularsieve structure with the secondary air source to effect oxidation of thedesorbed hydrocarbons and to cool the molecular sieve structure.